bobonker

Hi All:

Ok, I know about not pedaling backwards in the big-big combo (and not using the combo in general) due to extreme cross-chaining.

I got my SRAM Force build all done and it shifts flawlessly. I'm super happy with it and feel that is is pretty dialed in. This evening (with the bike on the stand) I had the chain on the big chainring up front and the small cog in the rear. This is the least extreme chain angle (I think). I spun the wheel backwards and not long after, the chain fell off in front starting at the bottom of the crank. It's totally repeatable.

Why? Should I accept this as normal? My old 6600 stuff never did this.

All of the parts are new. Chainrings don't look bent. Rear D hangar is straight. Rear D looks straight...I hope it is. It's brand new.

Thanks,
Bob

FBinNY 

Actually the straightest chain line is with the outer and the third or fourth sprocket, but that's beside the point. You should be able to backpedal without dropping the chain in most, if not all, of the normal gear combinations.

Since everything is new it could be simply a break in issue, and will go away after a while as the chain cleans up the chainring. It could also be a chain line issue whth the cranks a bit too far inboard, a lubrication problem (I have to say that, consider the source), or it could be that the RD is not quite vertical and the cage is a bit more outboard at the bottom.

I'd start by confirming that the chainline is on target, and if so give it a chance to break in a bit before looking at the RD alignment.

Al1943

Yes, this sounds like a slight chainline problem, not caused by the derailleur. The simple cure is to not pedal backwards in that combination. If your bottom bracket is a "conventional" type, internal bearings, you could shift the driveside bearing cup and spindle slightly to the right with a bottom bracket spacer. This is simple and easy to do but I think you should ride the bike and let everything settle in before you try a spacer.

bobonker

Ok, guys. I'll ride it for awhile and see if it cures itself. Thanks!!

Bob

bobonker

Oh BTW -- On the original Shimano setup, there was a thin (~1mm) spacer between the drive side bottom bracket cup and the frame. When I put the SRAM bottom bracket in, I initially left the spacer out because I thought it was specific to the Shimano BB. Without the spacer, there was play in the crank even though it was fully tightened. When I put the spacer back in, the play was gone, so I assembled it with the spacer.

The SRAM bottom bracket came with a pair of 2.5mm spacers, but it looked like those were for MTB use.

Bob

DannoXYZ 

Actually, when back-pedaling, the rear-derailleur is what guides the chain to the front-chainring. In the big-ring/small-cog combo, the rear-derailleur should be outboard of the chain-line and should move the chain off to the outside when back-pedaling.

FBinNY 

To say the RD guides the chain to the front sprocket is a bit of a stretch since it's a foot away. But yes, If the chain is coming off it would be to the outside since that's where the RD lower pulley should be in high gear.

But in normal riding the chain should run from the top of the outer sprocket unguided to the chainring (RD should touch it except during a shift) and stay on. So if the RD pulley is directly below the sprocket the angle should be the same and the chain shouldn't derail. The difference is chain tension and gravity which can overcome a bit of friction in upper (forward turning) engagement, but not lower in borderline cases.

Al1943

I assumed that the chain is coming off to the outside since it happens with the cranks turning backwards and he says that the chain comes off from the bottom, and the small cog is farther off the chainline than the big ring.

bobonker

Yep. It comes off to the outside of the large chainring starting from the bottom of the chainring.

Bob

reptilezs

gxp bbs use the non drive side to index the chain line. play in comes from the left crank arm not bottoming out and capturing the non drive bearing. sram also had a run on gxp bbs that were out of spec, no amount of removing/regrease/retorque got rid of the play. something to look into

Al1943

According to Sheldon Brown https://www.sheldonbrown.com/chainline.html, a double crankset should have a chainline 43.5 mm from the centerline of the bike. You could try measuring yours, from the outside of the down tube plus 1/2 the diameter of the down tube.
From the photo it appears that you have an outboard bearing bottom bracket. I did not know that you could use a BB spacer with that type. Since you already have I suppose you could try moving the spindle out with an additional spacer. But see how it measures first. Or quit worrying and go enjoy your bike.

bobonker

Sage advice right there. I'm on it.

Bob

FBinNY 

Probably applies to 90% of the issues raised in these forums, but not what folks usually want to hear.

Just got back in. 50° in New York on New Years Day, who could pass that up!

DannoXYZ 

It does guide it because the RD determines the angle the chain makes as it encounters the chainring on the bottom. The greater the angle away from 0-degrees, the more likely the chain will walk off the chainring.

Sure, the angle is less than with the rear-cogs, but RD doesn't have to sweep across 10 chainrings just to pull it off one.

Al1943

This is quite true. Maybe the OP should check the rear derailleur high limit screw and consider the RD hanger alignment.

FBinNY 

I wasn't arguing with the fact that the chain comes from the lower pulley, or that it's outboard, under the outer cassette sprocket. I was simply saying that IMO, at a foot away, it's a bit far to think of it as "guiding" anything.

Anyway the angle coming off an aligned RDs lower pulley is at the same angle as one coming off the respective rear sprocket to the top of the chainring, so it warrants some thought as to why a chain that doesn't fall off when pedaling forward falls off when backpedaling.

DannoXYZ 

Yes, even though the angle may be the same, the tension on the top-run of the chain is a lot higher due to pedaling-forces. So even if the edge of a bent chain was to encounter the top of a chainring tooth, the higher tension would force it down the tooth. Whereas on the bottom run, the much lower-tension allows the edge to stick to the tooth, ride on top of it and the next link lands outside of the chainring.

And the front-derailleur keeps the chain from moving laterally so much so as to move off the chainring unintentionally. This really is a problem on bikes where people remove the front-derailleur to use only a single chainring. Depending upon which gear the rear is on, the chain will fly off the inside or outside on the top-run as well. The solution to the single front-chainring is to leave a front-derailleur on or attach a chain-guide of some sort, such as double-chainring plates.

BTW, this seems like an odd issue. So I went to the bike in the garage and put it into the top-gear (big-ring/small-cog) and spun it backwards. WOW, in less than 1/2 a crank-revolution, the chain came off on the outside. I've had this bike for 20-years and never had this happen (probably because I never pedal backwards, that energy is better spent moving me faster in a forwards direction). I guess you can find peculiarities in mechanical configurations that can be made to malfunction. Kinda like how my 2" of toe-overlap with the front tyre has never caused me to crash, but I guess it's possible if I make various circumstances coincide just right.

Al1943

Oh man! You need to go buy a new bike right away!

FBinNY 

Danno, I don't know why we're debating something we agree on.

If you read the second sentence of my post which you quoted, where I quibble with the choice of the word "guide" you'll see that I say the exact same thing about the effect of tension and gravity making a difference between the top and lower chords.

But, even though I don't backpedal just for the hell of it, I insist that all my bikes can keep a chain on if I do. It's a matter of convenience, for example I might want to lift the starting pedal, coming off a stop (countless times per day on the commuter) or I might end up walking a bike backward, and don't want the chain to get independent on me.

deacon mark

I can spin mine backwards in any combination no issues. Now just read my post on the chainrub on the chainring. I really think you should be able to pedal backwards but what do I know?

bobonker

Update on this old thread:

I swapped out my trusty SRAM 12-27 "climbing" cassette for a Shimano 11-23 cassette.

With the Shimano cassette, the chain does not fall off when pedaling backwards in the big-small combo. The Shimano cassette requires the 1mm spacer behind it, so it looks like overall, it sits just a tad further away from the wheel than the SRAM cassette.

Not a huge deal, but it has caught me out once in 3 months. Was stopped somewhere after a long descent in the 50-12. Unclipped and rolled the back back a little. Chain fell off. $#@!!

Bob

fietsbob

Just dont back pedal... Forward ...